1. Field of the Invention
The present invention relates, generally, to hydrokinetic couplings used to transfer kinetic energy and, more specifically, to a torque converter having a clutch assembly which is operable to provide direct torque translation between the torque input member of a transmission through the turbine assembly at low engine speeds.
2. Description of the Related Art
In automotive applications, engine torque and speed are translated between a prime mover, such as an internal combustion engine, to one or more wheels through the transmission in accordance with the tractive power demand of the vehicle. Hydrokinetic devices, such as torque converters, are often employed between the internal combustion engine and its associated transmission for transferring kinetic energy therebetween.
As illustrated schematically in FIG. 1, torque converters 10 include impeller assemblies 12 operatively connected for rotation with the torque input from the internal combustion engine, a turbine assembly 14 fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly 16. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes which act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly 16 of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly 12 and turbine assembly 14. When the stator assembly 16 is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter.
Impeller, turbine and reactor blades may be either cast or stamped to a desired shape. Cast blades are most often employed for stators. In addition, cast blades are also employed in turbine and impeller assemblies in large industrial applications. Cast blades generally have varying thicknesses in the radial and axial directions and contoured working surfaces.
Stamped metal blades are typically cold worked in a form die such that they include ribs stamped thereon for rigidity and strength. Arcuate contours are also imparted to the stamped plate worked blades such that the blades have complex three dimensional working surfaces. For example, conventional turbine blades are curved between fluid inlet, the torroidal shell and core surfaces and the fluid outlet. This is referred to in the art as a "ruled surface" and is generated in production by the form die.
The shape and contour of the blades (blade form) are important considerations which can have significant effects on the capacity, torque ratio, efficiency and overall performance of the torque converter as well as overall powertrain efficiency. Due in part to this performance sensitivity, the contoured, stamped blades must be made with specific materials which have limited "spring back" characteristics such that there is very little or no deviation from the shape imparted to the metal in the form die.
Accordingly, the shape and contour of conventional blades in the related art must be tightly controlled during production. Such control and sensitive manufacturing procedures increase the cost of manufacturing the torque converter. Thus, there is a need in the art for simplified blades which have good performance characteristics and which can be manufactured at lower costs.
Torque converter performance characteristics can be measured by observing the relationships between torque ratio (T.sub.2 /T.sub.1), where T.sub.1 is the torque input to the converter and T.sub.2 is the torque output of the converter; speed ratio (N.sub.2 /N.sub.1), where N.sub.1 is the rotational speed input to the converter and N.sub.2 is the rotational speed output from the converter; efficiency; and K-factor curves. The K-factor is related to torque converter capacity and slip. In conventional torque converters, the K-factor typically shows a significant rise between speed ratios of 0.5-0.85 as compared to lower speed ratios of between 0-0.3. A rise in the K-factor curve has a negative effect on overall powertrain efficiency. Thus, it is highly desirable to increase or maintain powertrain efficiency by inhibiting the rise in the K-factor curve.
When the torque ratio is 1.0 or less, the stator assembly 16 will "free wheel" or spin in the direction of rotation of the impeller assembly 12 and the turbine assembly 14. The "coupling point" is a term used in the related art to describe the point where the torque ratio is 1.0. At the coupling point, there is no torque multiplication as described above. When there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (rpm input&gt;rpm output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
Conventional torque converters often employ clutches interposed between a torque input member and the turbine assembly which are engaged and "lock up" at high speed ratios of between 0.88-0.98. When the clutch is locked up there is a direct torque translation between the torque input member and the transmission through the turbine assembly. The locked up clutch eliminates the slip inherent with the fluid coupling and results in an efficiency gain for the torque converter. However, the transmission is usually shifted from first through second and sometimes into third and fourth gear before the clutch locks up. Depending on throttle position and shift strategy, this may occur at vehicle speeds of between 40 and 45 mph. Thus, conventional lock up clutches are generally engaged only after the torque converter has been operating as a fluid coupling limiting powertrain efficiency. Theoretically then, the earlier the clutch is engaged, significant output efficiency gains can be realized. However, as explained below, clutches in the related art are typically actuated at higher speed ratios to avoid translation of vibration noise, etc.
At lower engine speeds (and thus torque converter speed ratios) there typically exists significant drive line torsional vibration. When the clutch is disengaged, the torque converter acts as a fluid damper and absorbs, dissipates or otherwise fails to translate these vibrations.
Thus, clutches are typically only operated at higher engine speeds (and speed ratios) where the resonant vibration, noise, etc. are not as severe. Clutch assemblies of the related art also employ torsional dampers to further attenuate the vibration, noise, etc. that occur. However, torsional dampers add weight, cost and complexity to the clutch assembly and the torque converter in general.
Thus, there is a need in the art for a torque converter which includes a clutch assembly which is operable at lower engine speeds at the approximate coupling point of the torque converter which thereby avoids inherent slip and increases the efficiency of the torque converter. In addition, there is a need in the art for such a torque converter which does not translate drive line vibrations but which also does not require a torsional damper. Finally, there is a need in the art for such a torque converter which enjoys these improved operating parameters and which has reduced weight, cost and complexity as compared with such devices in the related art.
Continuously slipping bypass friction clutches are known in the related art to enhance the operating efficiency of hydrokinetic torque converters and are sometimes employed to address the problems identified above. These clutches are subjected to a continuously slipping operational mode. However, without adequate cooling by the operating hydraulic fluid in the converter, they can generate excessively elevated temperatures leading to catastrophic degradation of both the facing material and the operating hydraulic fluid. The hydraulic fluid is a special oil formulation commonly known as automatic transmission fluid or ATF.
To maintain functional design intent characteristics, two critical phenomena must be satisfied for best slipping clutch performance. One is the ability to efficiently conduct heat away from the clutch interface zone. The other is the ability to maintain a wetted interface zone avoiding potential areas of so-called "dry friction" that can produce erratic friction characteristics and excessive or uneven wear resulting in a significantly shortened clutch life. Moreover, a hydrokinetic torque converter presents specific restrictions on function that most other forms of wet clutches do not experience. For example, space limitation in the converter usually dictates a very limited number of interface zones of relatively large annular area with the most common typically having only one interface zone. A more advanced compact heat resistant design having multiple interface zones is disclosed in U.S. Pat. No. 5,337,867. In such converters with a continuous slip bypass clutch, the hydraulic circuit within the converter typically includes cavities that surround the outer diameter of the interface zone with high pressure hydraulic fluid and the inner diameter thereof with a low or zero pressure hydraulic fluid. This condition exists in concert with total assembly rotational velocity. The differential pressure between these cavities is modulated and utilized to apply the bypass clutch to control the slip speed in the clutch while fluid is circulated through the interface clutch zones from the high pressure cavity to the low pressure cavity to wet and cool the clutch friction surface interfaces. However, the volumetric flow of oil through the interface zones is typically restricted to very small values (e.g. not exceeding 1 gpm) because of the restrictions imposed on the hydraulic supply circuit serving the converter circuit and the slipping bypass clutch limiting the ability to both adequately wet and cool the clutch surfaces. Moreover, it is desired that the bypass clutch is capable of extended operational periods in a low velocity slip mode (e.g. 300-10 rpm relative speed) at high interface energy levels (watts/mm.sup.2) that result in extremely high heat generation at the respective clutch interface zones.
In an attempt to meet these objectives, various forms of groove or channel patterns in the clutch facing frictional liner material have been proposed such as radial grooves, cross-hatch grooves and a combination of radial and annular grooves. Examples of such prior art groove patterns are shown and described in U.S. Pat. No. 5,566,802 and incorporated herein by reference.
The groove patterns known in the prior art attempt to control the flow of ATF between the high pressure and low pressure cavities and across the friction liner for purposes of wetting with various degrees of success. The groove patterns are typically cut on the paper based frictional liner facing materials using a die. When some groove patterns are cut, they result in a segmentation of the friction liner material into muitiple pieces. The multiple pieces must then be assembled like a jigsaw puzzle onto the face of the friction plates. This takes time and is labor intensive, ultimately increasing the cost of the manufacturing process. Thus, there is a need in the art for a continuously slipping clutch equipped with friction liners having adequate grooving for wetting and cooling purposes but which are not segmented into multiple sections when die cut during a manufacturing process.
In addition to the efforts to maximize performance of the torque converter, there are also design considerations which dictate minimizing, to the extent possible, the space occupied by the torque converter in a transmission housing. However, these "packaging" design objectives often compete with optimum performance requirements. Torque converter packaging may be considered in relation to the ratio of the toroidal axial length L to the radial dimension of the torque converter expressed as R.sub.1 -R.sub.2, where R.sub.1 is the outer radius of the torroid and R.sub.2 is the inner radius of the torroid, measured from the axis of rotation 18 as shown schematically in FIG. 1.
This relationship, L/(R.sub.1 -R.sub.2), is known in the art as the "squash ratio". Reducing the squash ratio, i.e. minimizing toroidal axial length versus toroidal radial dimension, increases the "squash" of the torque converter and is desirable from a packaging standpoint. Conventional torque converters typically have squash ratios of approximately 1.0. However, and as a general matter, a reduction of the squash ratio has a negative effect on the K-factor curve and reduces the efficiency of the torque converter and the overall efficiency of the powertrain.
Thus, in addition to lowering the cost of manufacturing such turbine blades, there is a need in the art for a turbine blade which has less performance sensitivity, which can be employed in a torque converter having relatively more "squash" than torque converters in the related art, but which does not negatively effect the performance requirements of the torque converter.